As the price of petroleum continues to increase and as the availability of petroleum becomes more questionable ethanol is becoming increasingly more important as a source for hydrocarbon-based fuels and chemicals. Ethanol has long been produced by the well known fermentation process. Another more recent process of producing ethanol is the reaction of methanol with hydrogen and carbon monoxide (syn gas). This method has significant advantages because relatively inexpensive reactants are employed; however, it requires a catalyst for commercially viability. Therefore, there has been much attention given to the development of better catalysts for this very important reaction.
There are basically three significant and important parameters or criteria by which catalysts are judged: stability, activity, and selectivity. Stability relates to how long the catalyst remains functional before breaking down; activity relates to the amount of reactants the catalyst can convert per unit time; selectivity relates to the amount of desired product as opposed to undesired products that the catalyzed reaction forms. Generally catalysts that increase one of the parameters tend to have a detrimental effect on another of these parameters with the result that whenever any one of the other parameters is decreased the yield of desired product from the reaction generally suffers.
It has long been known that a water soluble cobalt catalyst and an iodine promoter will catalyze the reaction of methanol, hydrogen and carbon monoxide to produce ethanol. However, this catalyst system has often been characterized with low ethanol yields due to poor selectivity. There have been many attempts to improve this basic catalyst system. For example, U.S. Pat. No. 3,248,432 discloses the introduction of a phosphorus compound soluble in methanol to the basic catalyst system; U.S. Pat. No. 3,285,948 discloses the use of halides of ruthenium or osmium as second promoters in combination with the basic cobalt and iodine catalyst system; Netherlands Pat. No. 7606138 discloses the use of tertiary phosphines and nonpolar solvents in addition to the basic cobalt/iodide catalyst system and further states that the nonpolar solvent is crucial to the attainment of the high selectivity reported; and U.S. Pat. No. 4,133,966 discloses a four component catalyst system composed of cobalt acetylacetonate, a tertiary organo Group VA compound, an iodine promoter, and, as a second promoter, a ruthenium compound.
It is known that the selectivity of the reaction of methanol, hydrogen and carbon monoxide to ethanol can be increased by increasing the mole ratio of hydrogen to carbon monoxide; unfortunately this increase in the mole ratio results in poor catalyst stability as the cobalt tends to precipitate out as the metal. It is also general knowledge that the stability problem can be overcome to some extent by the use of a tertiary organo Group VA ligand. This use is shown, for example, in U.S. Pat. No. 4,133,966; however, this has created another problem for as one increases the level of the tertiary phosphorus compound in relation to the other catalyst components, the activity of the catalyst is decreased. This limits the total yield of ethanol which can be obtained. A method which can allow the use of higher amounts of Group V ligands without harming catalyst activity so that catalyst stability can remain high even though the hydrogen to carbon monoxide mole ratio is increased for purposes of higher selectivity, would be advantageous.